The invention relates to an exhaust-gas recycling device for an internal-combustion engine, especially a diesel engine, in which an exhaust-gas part stream can be returned to the combustion spaces of the engine by means of a line which can be shut off by a spring-loaded exhaust-gas recycling valve, the spring of the exhaust-gas recycling valve pressing against a diaphragm exposed to a vacuum dependent on engine-operating data, with the force directed counter to the vacuum, the valve being closed in the case of an excess of the spring force and the valve being opened ranging from partially to completely in the case of an excess of the force of the vacuum, depending on the amount of this excess.
A device of this type is known, for example, from German Patent Document DE 2,549,959 B.
The diaphragm installed in such an exhaust-gas recycling valve tolerates only a specific maximum temperature load by virtue of its material. If the admissible temperature limit is exceeded, there is the danger that the diaphragm will be damaged. This problem has hitherto been solved by setting, at any point of the engine in the engine housing, a temperature beyond which the vacuum acting on the diaphragm has been reduced, with the result that the exhaust-gas recycling valve has automatically closed under the still effective pressure of the spring. The disadvantage of this solution is that the measured temperature is merely an experimentally determined reference temperature for the temperature to be expected on the diaphragm. In this method, therefore, there is a relatively high degree of unreliability as regards a correct detection of the temperature of the diaphragm. This means, in turn, that, as a safeguard against the destruction of the diaphragm, the limiting temperature at the reference point has to be set so low that, even in the most unfavorable circumstances, the admissible temperature is not exceeded on the diaphragm itself. A measurement of the diaphragm temperature itself, so as to use this as a direct criterion for controlling or canceling the vacuum when the admissible diaphragm temperature is exceeded, involves an outlay which, as a rule, cannot be justified on economic grounds.
Starting from the above-explained background, an object on which the invention is based is to make it possible to close the exhaust-gas recycling valve at an inadmissibly high diaphragm temperature as simply as possible and in a way ensuring a complete utilization of the admissible temperature range. Furthermore, it will also be possible, at low outside temperatures, to keep the exhaust-gas valve closed even in those engine-operating states in which it would actually already have been opened as a result of the vacuum dependent on those operating data.
This object is achieved in that the force of the spring acting on the diaphragm of the exhaust-gas recycling valve is temperature-dependent.
The spring resting on the diaphragm is exposed, by way of this contact, to approximately the same temperature as the diaphragm, with the result that it is possible with high accuracy to adhere exactly to the upper limiting temperature diaphragm predetermined by virtue of its material.
Since it is mainly expedient to close the valve when and only when the upper limiting temperature which the diaphragm can tolerate is reached, it is appropriate to activate the temperature dependence of the spring only shortly before this limiting temperature is reached, whilst in the temperature range below it the spring force remains essentially unchanged.
To safeguard the diaphragm against overheating in a predetermined upper temperature range, the temperature dependence of the spring force must be designed in such a way that the spring force increases with an increasing temperature.
Moreover, for a lower temperature range the temperature dependence can be set so that, from a lower temperature limit value, the spring force likewise increases with a decreasing temperature. The last-mentioned setting is useful for starting and running up a cold engine, for example for starting an engine which is at temperature below 0 degrees Celsius. In these instances, in view of the engine-operating state it is sometimes desirable to prevent exhaust-gas recycling, even it would already have occurred per se according to the control dependent on the engine-operating conditions and usually taking place by means of a vacuum.
Under all circumstances, between an upper and a lower temperature range in which the spring force is temperature-dependent in the above-described way, there should be a middle temperature range in which the spring force remains independent of temperature, so that the valve can be controlled as a function of the engine-operating state solely by means of the vacuum applied to the diaphragm. Of course, the control dependent on the engine-operating state can also be obtained via any other control medium instead of by means of a vacuum.
The desired temperature dependence of the diaphragm spring can be achieved if the spring consists of a memory material which, as a result of material-property changes adjustable to specific temperature ranges, produces spring forces differing as a function of temperature.
Since, where memory materials are concerned, the temperature ranges in which material-property variations occur are, as a rule, relatively restricted, a plurality of spring elements consisting of differently set memory materials can be connected in series.
If a memory material is used as the diaphragm spring, it is recommended to employ a helical spring.
A temperature-dependent variation of the spring force can also be obtained by the use of bimetallic materials. The bimetallic materials here can be employed, for example, in the form of cup springs, if appropriately connected in series. The combination of a conventional helical spring of a spring force which is not temperature-dependent with bimetallic elements experiencing deformation as a function of temperature is also possible. In instances of such a combination, the bimetal should have as direct a contact with the diaphragm as possible, so as to possess temperature identity with the diaphragm. Springs made of memory material can also be combined with bimetallic elements.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.